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Full text of "The manufacture and properties of iron and steel"

304                         METALLURGY OF IRON AND STEEL.
passes except the last, but was then held at the finishing pass minutes, the result being that both pieces went through the finishing pass at the same temperature, which was about 750 C. I will designate as the "hot-rolled rail" the one which was rolled rapidly, but which was cooled down just before the finishing pass, and as the "'cold-rolled rail" the one which was rolled ai a lower temperature during the whole operation.
No. 34 represents the micro-structure of a portion of the hot rolled rail at a place very near the surface and "^o. 35 the structure of the cold-rolled rail at a similar place. It is evident that a superficial examination of photographs,-without any knowledge of certain fundamental conditions, might lead to the conclusion that ihe two methods'of rolling gave identical results, but the testimony of JSTos. 23 and 24 proves quite the opposite. No. 23 is from the center of the head of the hot-rolled rail and No. 24 from the center of the cold-rolled rail, and it is ciear that there is a radical and fundamental difference in the results, the reason for which is perfectly clear.
The finishing pass in almost every set of rolls does very little work, for it is unusual to have over ten per cent, of reduction upon the piece, oftentimes there being much less, while in all other passes, save one regulating the height, it is usual to have from twice to three times as much. Consequently the effect of the last pass does not penetrate to any great depth. Such a penetration is necessary if the grain is to be broken up, for the head of a heavy rail offers a thicker mass of metal than is found in almost any other structural shape, and the very fact that it is considered necessary to hold a rail before finishing proves that the grain needs to be' broken. If the rail is at a sufficiently low temperature the grain will not grow coarser as the rail stands, and the rail might as well be finished at once; but if it is at a high temperature and the grain is coarse, then it will do no good to hold it before the last pass, or to shower it with water, for this will merely perpetuate the coarse crystallization that exists. The holding of the rail therefore before the last pass is a delusion; it gives a lower finishing temperature and a low shrinkage, and it renders possible a very nice looking photograph from a piece of the outside skin, but it does not give any of the fundamental good qualities which should accompany such a finishing temperature, and which will accompany it if the temperature of the finishing pass is a true exponent of the rolling conditions. The